I. Field of the Invention
This invention relates generally to a cardiac pacing system in which the pacing rate is adjustable to meet physiologic demand and more particularly to a rate adaptive pacing system in which living cell tissue is used to sense metabolic need.
II. Discussion of the Prior Art
For approximately 25 years now, implantable cardiac pacers have been used to treat patients with defects in the cardiac conduction system, such as complete or partial heart block, bradycardia attributable to sick sinus node, atrial disease, A-V nodal disease, and even in cases of congestive heart failure. More recently, attempts have been made to more closely mimic normal heart operation. For example, attempts have been made to emulate the action of the S-A node and this has led to a class of cardiac pacers referred to as "rate adaptive pacers", "rate responsive" or sometimes "physiologic pacers". In these latter devices, some sort of a sensor is used to measure such things as blood temperature, blood oxygen saturation, body motion or activity, blood pH, respiratory rate, etc., and then use the sensed information to adjust the rate at which the pacer pulses are generated so as to accommodate the patient's level of activity.
In the introductory portion of the Koning et al Pat. No. 4,716,887, there is set forth a synopsis of several prior art patents relating to rate adaptive pacers and, especially, the particular physiologic parameters used heretofore for developing a rate control signal for an implanted pulse generator. Readers wishing additional background concerning the state of the prior art are referred to the Koning et al Patent and the references cited therein.
Most prior art pacemakers are prescribed for patients who have heart block and/or sinus node disease. Standard pacemakers pace either one chamber (atrium or ventricle), or both chambers (atrium and ventricle). Dual chamber pacemakers, commonly referred to as VDD and DDD pacemakers, are capable of tracking sinus node or atrial activity and then pacing the ventricle in synchrony. Such devices increase the pacing rate to meet the growing need for blood flow during periods of stress or exercise. Standard single chamber pacemakers, generally speaking, are not configured to adjust the pacing rate automatically, but rate responsive or rate adaptive pacemakers, both single and dual chamber, have been designed to be able to change pacing rate based on some direct or indirect indicator of stress or exercise. As set out in the Koning patent, various ones of these rate responsive pacemakers monitor a variety of conditions. None of them, however, truly mimics the sinus node which is the normal pacemaker or control center for the heart and the one true indicator of appropriate heart rate. The sinus node cells are responsive to both nerve impulses from the autonomic nervous system and to blood chemistry to set the heart rate. More particularly, the concentration of oxygen and carbon dioxide in the bloodstream as well as other chemical agents, such as drugs, hormones, etc., are integrated in the sinus node and affect the rate of depolarization/repolarization of the cells comprising that node.
Generally speaking, mechanical, electrical or chemical sensors all suffer from one or more serious disadvantages, typically that they lack physiologic sensitivity and specificity. As such, such auxiliary sensors tend to be either too slow or too fast and thus require additional prosthetic components. Unlike artificial sensors of the prior art, the present invention provides sensitive and specific responses to stress and exercise and does not rely on a purely prosthetic sensor which is oftentimes the weakest link in terms of reliability as well as adding complexity to the overall pacing system.